Problem: With pumpkins having large variations in size and shape, it is not easy to design something that will work on a particular fruit. In order to ensure the design will transfer correctly to the individual pumpkin, it would be useful to know where the skin of the pumpkin would be whilst working in the 3D design application.
Solution: Use 123D Catch to create a 3D mesh of the lucky pumpkin, open the mesh in the 3D design software, and use it to ensure the desired cuts will work as intended.
The technique I used works well with pumpkins, but it could also be used for machining into other organic or random shapes.
Step 1: Place reference marks on pumpkin and take photos
In terms of taking pictures for use in 123D Catch, I would like to give a little bit of advice. First of all, don't bother spending lots of time building contraptions to automate the picture taking process. I know there are some plans published all over the internet, but I think your time is more wisely spent by studying and understanding the excellent tutorials on the 123D website: http://www.123dapp.com/catch/learn
Catch is very capable and as long as you follow the advice in the tutorials, very good results can be achieved by just using a hand held camera on it's own. One thing I noticed is that a lot of peoples, also some of the guys (m/f) with the automatic-picture-taking devices, only take a series of picture in a circle at a single elevation. The best results, as advised in the tutorials, is achieved by taking pictures at two elevations.
Step 2: Create a 123D Catch
Step 3: Prepare 3D mesh for export
Step 4: Import 3D mesh into 3D CAD and align
As I was on a very strict timeline, I stuck with a competitor's CAD application I am more familiar with. Many 3D CAD packages are able to work in a similar way. I used the reference holes in the mesh in combination with photos to align the mesh in 3D space. The actual pumpkin was flat on one side, which I used for placing it. This proved very useful for alignment on the machine bed later on.
Step 5: Make a 3D model for the cuts
Initially I came up with the design for a scary pumpkin, but the boss told me to go for a friendly one...
Step 6: Generate G-code
Step 7: Align pumpkin on CNC machine bed
I actually did have two pumpkins. As it happened I did not have time to machine both of them. Note the difference in size between the pumpkins from looking at the attachments to this step. Although I did not actually machine the second one, the pictures show how I managed to adapt the same design to a different size.
Step 9: Manual finishing
Step 10: Conclusions
Now I managed to machine a simple pumpkin for this year, for next year I will have to raise the bar. A complicated surface model or do I add an indexing device for slowly rotating the pumpkin and therefore machine the entire circumference? Maybe I will just have to figure out a way to speed up the machining time and cut loads of pumpkins. So many possibilities and so little time...
O, before I forget, the actual pumpkin was stolen before it could see daylight. A few days later I found it smashed up, a bit further in the street. :-(